1,2-Dichloroethene [CAS 540-59-0] exists as a mixture of two isomers; namely, the trans isomer [CAS 156-60-5] and the cis isomer [CAS 156-59-2]. Both isomers are valuable compounds having a variety of uses. The isomers can be used separately or together in various proportions. It is reported that 1,2-dichloroethene can be used as a low temperature extraction solvent for organic materials, such as dyes, perfumes, lacquers and thermoplastics, as a solvent for the manufacture of rubber solutions, as a coolant in refrigeration plants, and as a chemical intermediate in the synthesis of other chlorinated hydrocarbon solvents.
1,2-Dichloroethene can be produced by the direct chlorination of acetylene at from approximately 40° C. to 80° C., by the reduction of 1,1,2,2-tetrachloroethane, by the pyrolytic dehydrochlorination of 1,1,2-trichloroethane, or as a by-product in the production of other C2 chlorinated hydrocarbons, e.g., trichloroethylene. However produced, the 1,2-dichloroethene product comprises a mixture of the trans and cis isomers, the quantity of which 1,2-dichloroethene product depending upon the conditions and process of manufacture, particularly when the 1,2-dichloroethene is recovered as a by-product. Mixtures of the trans and cis isomers of 1,2-dichloroethene typically exist as an equilibrium mixture, with the equilibrium concentration ratio of cis:trans being reported as being from approximately 2:1 to 5:1, depending upon the temperature and pressure of the mixture. At 825° C., for example, the equilibrium mixture contains about 55 percent of the cis-isomer; while at 975° C., the proportion of the cis-isomer falls to 52 percent. See, U.S. Pat. No. 5,051,536.
For some uses, trans-1,2-dichloroethene or a trans-rich mixture of the two stereoisomers is favored. Trans-1,2-dichloroethene is more reactive chemically than the cis-isomer, especially in 1,2-addition reactions. It also has a lower normal boiling point and a lower latent heat of vaporization than the cis-isomer. Consequently, the trans isomer or a trans-rich mixture is favored for extractions where it is desired to recover the 1,2-dichloroethene by distillation for recycle. The trans-isomer also has a lower viscosity than the cis-isomer, so that less energy is required for pumping the trans-isomer or trans-rich mixture.
For other uses, cis-1,2-dichloroethene or a cis-rich mixture of the two stereoisomers is favored. Since the cis-isomer has a lower melting point than the trans-isomer, the cis-isomer or a cis-rich mixture is better suited for use as an indirect heat transfer medium in refrigeration systems operating at low temperatures. The solubility of the cis-isomer in water at 25° C. is less than that of the trans-isomer, so that the cis-isomer or a cis-rich mixture of the stereoisomers is favored for some extractions where an aqueous phase is present. The lower chemical reactivity of the cis-isomer or a cis-rich mixture is also better suited where chemical stability is desired.
When the trans-isomer or a trans-rich mixture is required, it is desirable to be able to isomerize the cis-isomer to the trans-isomer. U.S. Pat. No. 5,051,536 describes isomerization of 1,2-dichloroethene isomers in the liquid phase in the presence of a free-radical initiator. In the embodiment described in Example 1 of the '536 patent, a feed comprising a mixture of 1,2-dichloroethene (cis and trans isomers) and other C1 and C2 chlorinated hydrocarbons is used to illustrate the conversion of the cis-isomer to the trans-isomer in the liquid phase at reflux conditions and in the presence of a free-radical initiator. When the trans-1,2-dichloroethene product is recovered as an overhead stream from this conversion reaction, the '536 patent suggests that it can be further purified by a subsequent distillation to yield a trans product with an assay of greater than 95 weight percent.
It has been observed that the overall recovery of trans-1,2-dichloroethene for the method described in the '536 patent is on the order of 45 to 60 percent, based on the amount of 1,2-dichloroethene isomers in the feed composition. It would, therefore, be desirable to increase the overall recovery of the trans-1,2-dichloroethene isomer from such a process.